Optimize `/sync` and history visibility (#2961)

Should fix the following issues or make a lot less worse when using
Postgres:

The main issue behind #2911: The client gives up after a certain time,
causing a cascade of context errors, because the response couldn't be
built up fast enough. This mostly happens on accounts with many rooms,
due to the inefficient way we're getting recent events and current state

For #2777: The queries for getting the membership events for history
visibility were being executed for each room (I think 185?), resulting
in a whooping 2k queries for membership events. (Getting the
statesnapshot -> block nids -> actual wanted membership event)

Both should now be better by:
- Using a LATERAL join to get all recent events for all joined rooms in
one go (TODO: maybe do the same for room summary and current state etc)
- If we're lazy loading on initial syncs, we're now not getting the
whole current state, just to drop the majority of it because we're lazy
loading members - we add a filter to exclude membership events on the
first call to `CurrentState`.
- Using an optimized query to get the membership events needed to
calculate history visibility

---------

Co-authored-by: kegsay <kegan@matrix.org>

1 files changed, 13 insertions, 14 deletions

diff --git a/syncapi/internal/history_visibility.go b/syncapi/internal/history_visibility.go
index 71d7ddd1..87c59e03 100644
--- a/syncapi/internal/history_visibility.go
+++ b/syncapi/internal/history_visibility.go
@@ -121,10 +121,7 @@ func ApplyHistoryVisibilityFilter(
 
 	// Get the mapping from eventID -> eventVisibility
 	eventsFiltered := make([]*gomatrixserverlib.HeaderedEvent, 0, len(events))
-	visibilities, err := visibilityForEvents(ctx, rsAPI, events, userID, events[0].RoomID())
-	if err != nil {
-		return eventsFiltered, err
-	}
+	visibilities := visibilityForEvents(ctx, rsAPI, events, userID, events[0].RoomID())
 	for _, ev := range events {
 		evVis := visibilities[ev.EventID()]
 		evVis.membershipCurrent = membershipCurrent
@@ -175,7 +172,7 @@ func visibilityForEvents(
 	rsAPI api.SyncRoomserverAPI,
 	events []*gomatrixserverlib.HeaderedEvent,
 	userID, roomID string,
-) (map[string]eventVisibility, error) {
+) map[string]eventVisibility {
 	eventIDs := make([]string, len(events))
 	for i := range events {
 		eventIDs[i] = events[i].EventID()
@@ -185,6 +182,7 @@ func visibilityForEvents(
 
 	// get the membership events for all eventIDs
 	membershipResp := &api.QueryMembershipAtEventResponse{}
+
 	err := rsAPI.QueryMembershipAtEvent(ctx, &api.QueryMembershipAtEventRequest{
 		RoomID:   roomID,
 		EventIDs: eventIDs,
@@ -201,19 +199,20 @@ func visibilityForEvents(
 			membershipAtEvent: gomatrixserverlib.Leave, // default to leave, to not expose events by accident
 			visibility:        event.Visibility,
 		}
-		membershipEvs, ok := membershipResp.Memberships[eventID]
-		if !ok {
+		ev, ok := membershipResp.Membership[eventID]
+		if !ok || ev == nil {
 			result[eventID] = vis
 			continue
 		}
-		for _, ev := range membershipEvs {
-			membership, err := ev.Membership()
-			if err != nil {
-				return result, err
-			}
-			vis.membershipAtEvent = membership
+
+		membership, err := ev.Membership()
+		if err != nil {
+			result[eventID] = vis
+			continue
 		}
+		vis.membershipAtEvent = membership
+
 		result[eventID] = vis
 	}
-	return result, nil
+	return result
 }